Joint replacement surgery seeks to replace some or all of a natural joint with prosthetic components so as to provide long-lasting function and pain-free mobility.
For example, in the case of a prosthetic total hip joint, the head of the femur is replaced with a prosthetic femoral stem component, and the socket of the acetabulum is replaced by a prosthetic acetabular cup component, whereby to provide a prosthetic total hip joint.
In the case of a prosthetic total knee joint, the top of the tibia is replaced by a prosthetic tibial component, and the bottom of the femur is replaced by a prosthetic femoral component, whereby to provide a prosthetic total knee joint.
The present invention is directed to orthopedic prostheses for restoring the knee joint and, in particular, to improved prosthetic tibial components.
Looking now at FIG. 1, there is shown a prior art prosthetic total knee joint 5 which generally comprises a prosthetic tibial component 10 secured to the top end of a resected tibia 15, and a prosthetic femoral component 20 which is secured to the bottom end of a resected femur 25.
A typical prior art prosthetic tibial component 10 is shown in greater detail in FIGS. 2 and 3. Prior art prosthetic tibial component 10 generally comprises a metal base construct 30 and a polyethylene bearing construct 35.
More particularly, metal base construct 30 generally comprises a baseplate 40 having a top surface 42 and a bottom surface 43, a stem 45 and a plurality of posts 50 descending from bottom surface 43 of baseplate 40 and into resected tibia 15, and a plurality of screws 55 passing through baseplate 40 and into resected tibia 15. Baseplate 40 preferably has a peripheral profile which generally matches the peripheral profile of the resected tibia 15. Metal base construct 30 also comprises a pair of locking rails 60 fixed to the top surface 42 of baseplate 40 and defining a groove 65 therebetween, and a pair of end walls 70 connected to top surface 42 of baseplate 40. Preferably bottom surface 43 of baseplate 40 (and, optionally, stem 45 and/or posts 50) comprises a porous material so as to allow bone ingrowth into baseplate 40 (and/or stem 45 and/or posts 50), whereby to facilitate osseo-integration of baseplate 40 (and/or stem 45 and/or posts 50) with resected tibia 15 over time. Additionally and/or alternatively, baseplate 40 and/or stem 45 and/or posts 50 may be fixed to resected tibia 15 with bone cement.
Polyethylene bearing construct 35 comprises a sculpted upper surface 71 having a central ridge 72 which separates a pair of scalloped seats 73 for receiving the condyles (either natural or prosthetic) of the lower femur. Polyethylene bearing construct 35 also comprises a flat bottom surface 75 having a recess 80 in which is disposed a tongue 85. Tongue 85 is sized to slidingly fit in the groove 65 which is defined by locking rails 60 of metal base construct 30 (FIG. 3), whereby polyethylene bearing construct 35 may be slidingly secured to locking rails 60 of metal base construct 30. Note that end walls 70 of locking metal base construct 30 act as stops for polyethylene bearing construct 35 when tongue 85 of polyethylene bearing construct 35 is advanced into the groove 65 which is defined by locking rails 60 of metal base construct 30.
In use, the top end of tibia 15 is resected, and metal base construct 30 is secured to tibia 15, i.e., by advancing stem 45 and posts 50 into resected tibia 15 until bottom surface 43 of baseplate 40 is seated against resected tibia 15. Note that the parallel dispositions of stem 45 and posts 50 facilitates advancement of stem 45 and posts 50 into the resected tibia. Next, screws 55 are advanced through baseplate 40 and into resected tibia 15, whereby to secure metal base construct 30 to resected tibia 15. Then polyethylene bearing construct 35 is locked onto metal base construct 30, e.g., by sliding tongue 85 of polyethylene bearing construct 35 into the groove 65 which is defined by locking rails 60 of metal base construct 30 until polyethylene bearing construct 35 engages end walls 70 of baseplate 40. Thereafter, the knee joint is reduced, allowing the condyles (either natural or prosthetic) of the lower femur to settle into the scalloped seats 73 of polyethylene bearing construct 35.
Unfortunately, in some patients, the natural geometry of the knee is such that there may be some degree of misalignment between the condyles (either natural or prosthetic) of the lower femur and the scalloped seats 73 of polyethylene bearing construct 35 of the prosthetic tibial component 10. Specifically, the anterior-posterior centerline of the condyles (either natural or prosthetic) of the lower femur, and the anterior-posterior centerline of the scalloped seats 73 of polyethylene bearing construct 35 of the prosthetic tibial component 10, may be angularly offset from one another. As a result, the condyles (either natural or prosthetic) of the lower femur may not seat properly in the scalloped seats 73 of polyethylene bearing construct 35 of the prosthetic tibial component 10. This may occur at some or all of the extent of flexure of the knee. This mis-seating of the condyles (either natural or prosthetic) of the lower femur in the scalloped seats 73 of polyethylene bearing construct 35 of the prosthetic tibial component 10 can lead to reduced stability of the knee in both static and dynamic conditions, and can lead to excessive wear of the polyethylene bearing construct 35 over time. In addition, this mis-seating of the condyles (either natural or prosthetic) of the lower femur in the scalloped seats 73 of polyethylene bearing construct 35 of the prosthetic tibial component 10 can lead to early loosening of the prosthetic tibial component, or to early loosening of the prosthetic femoral component, or both, and/or it can result in poor bone coverage leading to bone subsidence.
Thus there is a need for a new and improved prosthetic tibial component for a knee joint prosthesis which can provide for better alignment between the condyles (either natural or prosthetic) of the lower femur and the scalloped seats of the polyethylene bearing construct of the prosthetic tibial component.